The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
Automatic transmissions typically include a hydraulic circuit or system that uses a hydraulic fluid, such as oil, for various purposes throughout the transmission. For example, the hydraulic system directs oil to various transmission components to act as lubrication between moving components, to act as a cooling system for dissipating waste heat, and to act as a hydraulic control system for actuating various devices. One typical device that is hydraulically actuated in an automatic transmission is known as a rotating clutch. Rotating clutches are located within a rotating portion of the transmission and are employed to selectively transmit torque when hydraulically actuated. Rotating clutches work in combination with other torque transmitting devices and with various gear sets to provide a plurality of forward and reverse gear ratios.
While the hydraulic system for providing hydraulic fluid to the components of the transmission is useful for its intended purpose, it is possible that the hydraulic fluid can entrain air during operation of the transmission or that air has accumulated within the hydraulic circuit when the transmission has not been operating. The aeration of the hydraulic fluid may occur at various locations within the hydraulic circuit, such as when the hydraulic fluid is pumped from a sump where the hydraulic fluid is stored. Entrained or trapped air within the hydraulic fluid can have several negative effects within the transmission. For example, entrained or trapped air can reduce the ability of the hydraulic system to dissipate waste heat and thereby reduce the efficiency of the cooling system within the transmission. Additionally, entrained or trapped air that builds up within a rotating clutch can affect the timing of hydraulically actuating the clutch. Mistiming of a clutching event can in turn lead to engine runaway or flare.
Various methods have been employed in an attempt to address and mitigate the negative effects of entrained air present in the hydraulic fluid. For example, bleed holes have been employed in rotating elements to attempt to bleed the trapped air and air buildup from the hydraulic circuit. Additionally, various control strategies have been employed to engage the clutches at appropriate times in order to pressurize the circuit in order to purge the accumulated buildup of air. While these methods are marginally useful for their intended purpose, there is room in the art for a system of removing the entrained air from the hydraulic fluid in an automatic transmission.